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Linear motion goes plastic

Linear bearings, also called linear guide systems, have been used in the U.S. since the early 1900s, most commonly in printing and textile industries. Previously limited to steel ball bearings, now more options — including plastic and ceramic bearings — are available. During the last 20 years, the precision and performance of plastic bearings in particular have improved significantly so they now withstand the rigors of demanding applications just as resolutely as standard steel ball-bearing systems.

Plastic linear guide systems work through the use of sliding elements. These elements provide higher acceleration speeds than recirculating ball bearing systems. What are the physical reasons behind this? Because there are no moving parts in a linear guide system, nothing mechanical takes place. Once the plastic sliding element starts to move, it's on its way. In contrast, ball bearings must accelerate and then push against one another to begin motion. It's the same with deceleration: The balls must lose speed slowly, or risk a skidding motion, banging together and eventually flattening the ball and damaging the shaft.

Theoretically the speed of a linear guide system with plastic sliding elements is unlimited. The upper limit depends on the machine itself: What the drive can handle, how fast the motor can go. For these systems, the static friction coefficient µ is around 0.16 and the dynamic friction coefficient is 0.13. On average they can run up to four meters per second — though applications with speeds to ten meters per second have been successful.

This makes plastic bearings well suited for quick start-and-stop applications.

Sliding-element linear bearings are often self-lubricating; these bearings have a tribological design that includes a base material, fibers for reinforcement, and a dry lubricant. The movement of the bearing causes the dry lubricant to transfer onto the shaft, making it self-lubricating. In fact a rougher shaft, rather than one that is polished, is recommended to draw out the lubrication.

Sizing procedures

The load to be carried and the speed at which it needs to travel dictate what size, type, and style of linear motion system is required. The first step in selecting a plastic linear bearing is to determine how much weight is needed to transfer from Point A to B in the application. It's important to be aware of the load's position (horizontal, lateral, or vertical) with respect to where the linear bearing system is located. The further the distance of the load from the bearings, the more the required load capacity increases. (We'll soon explore this further.) The static load capacity of a linear bearing is generally given in psi. To calculate the static load capacity, the radial force is divided by contact surface area.

PV values (surface pressure multiplied by speed) result from moving masses. They determine the speed at which the linear guide system can operate. Speeds of more than 10 m/sec, which are often desired in today's high-cycle time manufacturing equipment, are easily achieved by plastic sliding elements. These speeds are achieved by eliminating any rolling elements and by utilizing low-friction shafting materials, such as hard anodized aluminum.

There is also relationship between maximum surface speed and load. Lighter-weight applications allow for greater running speeds. In other words, with decreased surface load, higher speeds can be obtained.

The friction between a bearing and its rail is the product of the coefficient of friction and the normal force pushing the two objects together — in most cases, the weight being carried. However, more significant than the maximum speed is the average speed per cycle time since the temperature of the bearing system is more affected. Therefore, to calculate the suitability of the linear bearing system, the average surface speed should be determined. In applications with intermittent cycles, the highest average surface speed is taken over a time period of 10 to 30 minutes.

Some linear guide systems take more power to move than others. Due to the higher coefficient of friction of a plastic sliding element in comparison to ball bearing guides, a higher driving force may be required. Rolling elements don't really have a coefficient of friction, though they're sometimes listed at 0.001 to 0.002. Again, for plastic bearings the coefficient of dynamic friction is 0.13. Larger bearing clearances (the space between the bearing and shaft) and having the smallest permanent load possible improve the friction and wear properties of plastic linear guide systems.

Friction values for plastic linear components improve over time. Upon start-up, friction values are at their highest point. Then after a brief break-in period, the coefficient of friction falls to a constant level, where it remains for the lifetime of the bearings. How is this achieved? The self-lubricating effect of plastic bearings allows for a microscopic transfer of bearing material to the peaks and valleys of the shaft or rail.

The 2:1 Rule

When using plastic linear bearings, the 2:1 Rule must be observed: If the distance of the driving force to the fixed bearing is more than double the distance between the two bearings, then the bearing can theoretically bind or cause a chattering movement at any static friction coefficient greater than 0.25.

The further the drive force is away from the guide bearing, the larger the wear rate and the higher necessary drive forces become. The 2:1 principle is not a result of the load or the driving force; rather it is a product of the friction of the sliding element. There are many different ways to ensure a design obeys this rule, including the addition of bearings, increasing the distance between bearings, or using a larger linear system.

Applications and conditions

Applications involving dirt, dust, sand, debris, and other abrasive media can clog and even seize bearings. A $50,000 machine can be damaged by an $8 bearing; the motor or drive system can also be compromised.

With ball bearing technology, these external conditions can prove even more damaging than with plastic. If a rolling element becomes scratched, the notch continues to widen if the balls keeps wearing over it and eventually the balls get damaged and can't roll through it anymore. In harsh environments, they often require additional equipment to protect the integrity of the bearing: Seals, wipers, and bellows. This adds cost to ball-bearing systems, with no 100% guarantee that the system is safe from environmental degradation. These protective elements are not necessary for self-lubricating linear guide systems.

To further illustrate the difference: If a pothole opens in a road, it widens as cars travel over it until eventually the tires — the rolling element in this situation — are ruined. On the other hand, a snowplow slides right over the pothole unaffected, just as a linear sliding element in its environment. In short, scratches and nicks form with them, but are inconsequential.

Maybe more obvious is the way plastic linear bearings stand up to wet or caustic environments. Water and chemicals can wash oil off steel linear bearings, opening up the potential for metal-to-metal contact. (This can lead to expensive maintenance and replacement costs.) Because self-lubricating bearings are made of plastic, they are immune to this threat.

Maintenance tips

In general, no quality standards exist for linear guide systems. For clean room or FDA applications, it's beneficial to use a self-lubricating bearing; this eliminates any risk of contamination. Self-lubricating guides are also the easiest system to maintain.

If using standard linear bearings, remember to keep them constantly lubricated — never operate while dry. If oil or grease is absent then high wear and failure will result.

Always mount the rail to a plane (flat) surface. Also make sure the rail is fully supported along the bottom if at all possible. Though the larger clearances of plastic bearings help offset the need for the most precise installation, proper alignment is critical to the smooth motion of all linear bearing systems.

Heed friction considerations. Design the application within the maximum allowable distance (to review, dependent on bearing length to load and drive distance.) Use the 2:1 Rule for self-lubricating linear bearings, and a 15:1 Rule for ball bearings. Do not purchase a motor insufficient to handle your drive application; make sure the motor force can overcome bearing friction while carrying loads.

Whenever possible, consult with a technical specialist from a linear bearing manufacturer to determine overall bearing system life. Because increased precision translates into higher cost, it's important to be specific and honest about the loads and speeds a linear guide system in design might actually experience. This can take very little time and effort, but can save a lot of money long-term.

As with any product or machinery, components are likely to fail in extreme environments, or with abuse and neglect. If regular maintenance will be difficult, choose a maintenance-free solution.

Predictions should be made as to when a bearing will need replacing for a given application, and how much wear will result from given application parameters. For standard applications linear bearings can often outlive the machine's required life.

Load capacity comparison

TYPE

SIZE

STATIC LOAD CAPACITY

Round

¼ to 2 in.; 6 to 50 mm

from 950 to over 10,000 lb per bearing

T-Profile Rails

9 to 30 mm

from 900 to over 3,000 lb per bearing

Miniature Profile Rails

17 to 80 mm

from 12 to 225 lb per bearing

Static load capacity depends on the size and type of bearing. Listed here are a few typical sizes.